1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
An exposure apparatus which projects and transfers a circuit pattern formed on a mask (reticle) onto a substrate such as a wafer via a projection optical system has conventionally been employed to fabricate a fine semiconductor device or a liquid crystal display device such as a semiconductor memory or logic circuit. The minimum dimension (resolution) of a pattern which can be transferred by the exposure apparatus is proportional to the exposure light wavelength, while it is inversely proportional to the numerical aperture (NA) of the projection optical system. In view of this, the shorter the wavelength and the higher the NA, the better the resolution. Along with the recent demand for micropatterning semiconductor devices, higher resolutions are becoming necessary.
Immersion exposure is receiving a great deal of attention as a technique of increasing the NA of the projection optical system. The immersion exposure further increases the NA of the projection optical system by using a liquid as a medium that fills the space under the projection optical system on the wafer side (image plane side). The numerical aperture NA of the projection optical system is given by NA=n·sin θ where n is the refractive index of the medium that fills the space under the projection optical system on the wafer side. A conventional exposure apparatus in which the space between a projection optical system and a wafer is filled with air (n=1) has an NA of 1 as a limit value. To improve this situation, the space between the projection optical system and the wafer is filled with a medium (liquid) having a refractive index higher than that of air so as to increase the NA of the projection optical system to that matching the refractive index of the medium. This makes it possible to provide exposure apparatuses which meet an increasingly growing demand for improving the resolution.
The control of an exposure apparatus requires monitoring various physical quantities to manage the apparatus state. For example, the exposure light quantity and imaging position are measured by monitoring actual exposure light having passed through a projection optical system, and are used to control the apparatus. Therefore, a light quantity sensor is required to have the capability to monitor physical quantities in an immersed state, that is, at a maximum NA.
The exposure apparatus uses, as the light quantity sensor, a photoelectric conversion device such as a photodiode which converts a light quantity into an electrical signal. In general, a photodiode is relatively susceptible to humidity. To prevent an immersion material from entering the photodiode, it is necessary to block the immersion material by inserting a transmission substrate on the image plane side of the projection optical system.
However, as the transmission substrate is inserted on the image plane side of the projection optical system, the light quantity sensor receives exposure light having passed through the projection optical system via the transmission substrate. In this case, light having an NA that exceeds 1 cannot reach the light receiving surface because it is totally reflected by the exit surface of the transmission substrate. To solve this problem, there is proposed a technique of preventing light having an NA that exceeds 1 from being totally reflected by the exit surface of the transmission substrate by bringing the transmission substrate into optical contact with a planoconvex lens. There are also proposed techniques of preventing light having an NA that exceeds 1 from being totally reflected by the exit surface of the transmission substrate in the following way. That is, the exit surface of the transmission substrate is designed as a diffusing surface, light is guided to the light receiving surface by reflecting it on the side surface of a cylindrical prism, or an optical fiber is used. Patent references are Japanese Patent Laid-Open Nos. 2005-268744, 2005-175034, and 2005-223275.
Unfortunately, the prior arts pose the following problems.
In the conventional technique of condensing light using a planoconvex lens, a measurement error due to the angle characteristic of the light receiving element is relatively large because the light emerging from the transmission substrate enters the light receiving surface while maintaining almost the same angle as the exit angle with respect to the transmission substrate. To solve this problem, the incident angle of the light with respect to the light receiving surface may be decreased by further inserting a plurality of lenses into the subsequent stage of the planoconvex lens. This, however, increases the number of lenses, resulting in a complex structure. When the angle of the light emerging from the planoconvex lens is relatively large, the area of the light receiving surface (light receiving element) of the light quantity sensor must be increased. This makes it difficult to ensure a space for accommodating the light quantity sensor and manufacture it. It is also possible to decrease the distance between the transmission substrate and the light quantity sensor so as not to increase the area of the light receiving surface. However, in an illumination mode in which a coherent factor σ of an illumination system is low (low σ), the incidence area of the light receiving element is narrowed down so that energy is concentrated on a specific portion, leading to a poor durability of the light quantity sensor.
In the conventional technique of guiding light to the light receiving surface by reflecting it on the side surface of a cylindrical prism, the light reaches the exit surface of the cylindrical prism while maintaining the same NA as that with which it enters the prism. For this reason, high-NA light is totally reflected by the exit surface of the prism without being transmitted through it. To solve this problem, the exit surface of the cylindrical prism may be brought into optical contact with the light receiving element, or the cylindrical prism may have a curved exit surface. However, these arrangements pose the following problems.
The arrangement in which the exit surface of the cylindrical prism is brought into optical contact with the light receiving element is impractical because they are likely to separate from each other. This is because a cylindrical prism and light receiving element are generally made of different materials, which come into optical contact with each other with a relatively weak bonding strength. The arrangement in which the cylindrical prism has a curved exit surface is also impractical because the light emerges from the prism and enters the light receiving surface while maintaining almost the same NA as that with which it enters the prism. This poses problems that a measurement error due to the angle characteristic of the light receiving element increases, or that the size of the light receiving element of the light quantity sensor must be increased.
The conventional techniques of designing the exit surface of the transmission substrate as a diffusing surface and using an optical fiber result in difficulty in manufacturing an optical element and in a complex structure.